It's unclear based on the timestamp on the Tweet and the
accompanying post what show exactly this agent was
monitoring. Since Glenn Beck only
just started a few minutes ago it can't be that.

The post with the screengrab of the Tweet went up at 3:34, which
is Shep Smith's hour. Shep doesn't really blather, and
today is was broadcasting live from the Mississippi
floodzone. Also of all the people one imagines the Secret
Service would be monitoring on Fox News, Shep seems the least
likely.

The mostly likely suspect is obviously Fox & Friends, but
again who's to say. Maybe they accidentally tuned in to
America Live and caught guest host Martha MacCallum talking to
the "Medical
A Team Expert" about Schwarzenegger. There was definitely
blathering.

Update: Time's
Amy Sullivan RT'd it and judging from her timestamp it does
in fact look like the Secret Service was tasked with watching
America Live...with Megyn Kelly still out on maternity leave
there is definitely a lot more blather. What particular
part of the blather the Secret Service was monitoring remains
unclear.